Communication services become increasingly diverse with widespread use of information processing and information communication technology and in particular, development of mobile communication such as mobile phone is remarkable. Currently, 3GPP (Third Generation Partnership Project) is working on standardization of the world standard “IMT (International Mobile Telecommunications)-2000” of a third-generation (3G) mobile communication system drafted by ITU (International Telecommunication Union). “LTE (Long Term Evolution)”, which is one of data communication specifications drafted by 3GPP, is a long-term advanced system aimed at fourth-generation (4G) IMT-Advanced and is also called “3.9G (super 3G)”.
LTE is a communication mode based on an OFDM modulation method and adopts OFDMA as the radio access method of a downlink. OFDM (Orthogonal Frequency Division Multiplexing) is a multi-carrier method by which a plurality of pieces of data is assigned to frequency sub-carriers that are “orthogonal”, that is, do not interfere with each other and can convert each sub-carrier on a frequency axis into a signal on a time axis for transmission by performing inverse FFT (Fast Fourier Transform) for each sub-carrier. Transmission data is transmitted by being distributed to a plurality of carriers whose frequencies are orthogonal and thus, OFDM is characterized in that the band of each carrier becomes a narrow band, the efficiency of frequency utilization is very high, and delay distortion (frequency selective fading disturbance) is resisted thanks to multi paths. OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access scheme in which, instead of all sub-carriers of an OFDM signal being occupied by one communicating station, a set of sub-carriers in the frequency axis is assigned to a plurality of communicating stations so that sub-carriers are shared by the plurality of communicating stations. If a plurality of users each use different sub-carriers or different time slots (that is, division multiplexing in a frequency direction and a time direction), communication can be performed without interference.
3GPP supports a bandwidth close to 100 MHz in a standard specification “LTE-Advanced”, which is a further development of LTE for a fourth-generation mobile communication system, and aims for realization of the peak speed of 1 Gbps at the maximum. A space division multiple access scheme in which radio resources on spatial axes are shared by a plurality of users like, for example, multi-user MIMO (MU-MIMO) or SDMA (Space Division Multiple Access) is regarded as very likely.
Moreover, relay technology is examined for LTE-Advanced to improve throughput at cell edges. The relay technology here is a mechanism by which a relay station (RS) is installed in an area (cell) of a base station connected to a core network to allow hopping communication between the base station and the relay station. If the communication speed is 1-2 Mbps or so, the modulation method such as BPSK (Binary Phase Shift Keying) and QPSK (Quadrature PSK) can be applied and a necessary SNR (Signal-to-Noise Ratio) is permitted even if the SNR is low at cell edges. In contrast, to obtain the communication speed of 100 Mbps or more, it is necessary to maintain the SNR high throughout the cell. Moreover, a higher operating frequency increases transmission losses and is sensitive to fading so that a coverage area of a base station deteriorates. Performance of a single base station falls at cell edges and a relay station compensates therefor.
In a downlink, a relay station amplifies a received signal from a base station and then transmits the received signal to a mobile station. With a signal being relayed, the SNR can be made larger when compared with a case when the signal is directly transmitted from the base station to the mobile station. In an uplink, on the other hand, the relay station can maintain the SNR high by receiving a signal from the mobile station and transmitting the signal to the base station (down-bound radio access from a base station (BS) toward a mobile station (MS) is called herein as a “downlink” and up-bound radio access from the MS to the BS as an “uplink”).
For example, a cellular system in which a base station assigns resources to terminals, transmits a downlink signal in the current time slot, and receives an uplink signal from terminals via a relay station in the next time slot, the relay station receives a downlink signal from the base station and an uplink signal from terminals in the current time slot and transmits the received downlink signal to the terminals and the received uplink signal to the base station in the next time slot, and the terminal transmits an uplink signal in the current time slot and receives a downlink signal via the relay station in the next time slot (see, for example, Japanese Patent Application Laid-Open No. 2008-22558).
The mode in which a relay station relays a signal between a base station and a mobile station can be classified into the following two types based on how a received signal is transmitted.
The first type is a mode called “Amplify-and-Forward (AF)” in which a relay station retransmits a received signal from a base station after amplifying the signal unchanged as an analog signal. In the AF mode, it is difficult for the mobile station to improve the SNR and thus, it is necessary for the relay station to relay by using a region in which signal strength is sufficiently large. Moreover, there is a feedback path between a transmitting antenna and a receiving antenna so that consideration must be given to prevention of oscillation. An advantage of the AF mode is that there is no need at all to improve the communication protocol.
The second type is a mode called “Decode-and-Forward (DF)” in which the relay station performs digital processing on a received signal from the base station and then amplifies and transmits the received signal. That is, the relay station converts the received signal from the base station into a digital signal by the AD conversion, performs decode processing such as an error correction on the signal, encodes the signal again, and converts the signal into an analog signal by the DA conversion before amplifying and transmitting the signal. According to the DF mode, the SNR can be improved by a coding gain. Further, an issue of a signal turnaround into between the transmitting antenna and the receiving antenna can be avoided by a signal converted into a digital signal being stored in a memory and the signal being transmitted in the next time slot by the relay station. Oscillation can also be suppressed by changing the frequency, instead of the time slot being changed for transmission and reception.
In LTE-Advanced, which is a future network of 3GPP, the DF mode capable of improving the SNR rather than the AF mode is more likely to be mainstream.
In LTE and LTE-Advanced, a reduction in communication delay is demanded and more specifically, reducing the delay between users to 50 millisecond or less is demanded. Thus, when relay technology is introduced, an issue of delay caused by the mediation of a relay station needs to be sufficiently considered.
While the DF-type relay mode improves the SNR by a coding gain, a delay caused by decoding and recoding is significant. Thus, a method by which the AF type that causes less delay is used for channels in which a delay demand is severe and the DF type is applied to channels in which a delay demand is not severe is proposed.
If relayed in the DF type relay mode by changing the time slot by time division to avoid interference, the delay increases in time slot. The delay when a relay station recodes and transmits a received signal is frequently aligned with a delay of one subframe or time slot. This is because if a relay station should be introduced while maintaining downward compatibility of LTE, such delimitation is easier to maintain compatibility. One subframe is a delimiter of an uplink and a downlink of TDD and thus is easier to adopt as the unit of delay of a relay station.
In LTE, intercell interference coordination (ICIC) is proposed to reduce an influence of interference between adjacent cells of the same channel.
The ICIC can be realized by, for example, a fractional frequency repetition combining a one-cell frequency repetition and a multi-cell frequency repetition.
Each cell is divided into a center region inside the cell close to a base station and a boundary region at cell ends apart from the base station. While a “central frequency” assigned to communication between the base station and the mobile station in the center region competes with that of adjacent cells (that is, a one-cell frequency repetition), interference between cells is avoided by controlling transmission power small enough so that a signal reaches only within a center region. On the other hand, it is necessary to transmit a signal large enough so that the signal reaches the boundary region and interference between cells is avoided by mutually different “boundary frequencies” being used by boundary regions of adjacent cells (that is, a multi-cell frequency repetition). Moreover, instead of all sub-carriers of an OFDM signal being occupied by one mobile station, sub-carriers of the central frequency are assigned to mobile stations near the base station and those of boundary frequencies to mobile stations apart from the base station so that sub-carriers are shared by a plurality of mobile stations to implement multiple access (OFDMA).
If, however, the relay technology is introduced into a cellular system in which the ICIC is adopted, there is the possibility that a signal retransmitted from the relay station may interfere with adjacent cells. This is because while the relay technology improves throughput at cell edges through the mediation of the relay station, the introduction thereof is equivalent to increasing power in the vicinity of cell edges. The relay station is, when compared with the base station, closer to cell edges, which increases the possibility of interfering with the central frequency of adjacent cells. Conversely, the possibility of the relay station being interfered with by the central frequency of adjacent cells also increases.
Even if the base station transmits a signal of the boundary frequency with strong power, interference with the central frequency of adjacent cells is small because the strong power sufficiently attenuates at cell edges. In contrast, the relay station transmits the boundary frequency closer to cell edges and thus, it is extremely likely that reception of mobile stations using the central frequency of adjacent cells is interfered with.
On the other hand, while the base station receives a signal using the boundary frequency from a mobile station in the boundary region, interference with the central frequency of adjacent cells is small. This is because base stations of adjacent cells and the base station of the local cell are sufficiently apart from each other and the base station controls transmission power at the central frequency to a low level. Reception of relay stations located in the boundary region is similarly considered to be without issues.
In short, if relay technology is introduced into a cellular system, a mobile station located in the boundary region uses the boundary frequency of the local cell, which is to be the central frequency of adjacent cells and therefore, an issue is considered to arise in reception of the mobile station.